Weighing refers to the determination of the force that gravitation exerts upon an object. It is commonly achieved by using a balance, scale, or other mechanical device.
Weighing is needed in the manufacturing industry (e.g., for weighing the raw materials used in fabrication and the products of the fabrication), the goods delivery industry (e.g., for weighing the goods in a delivery truck), the asset control industry (e.g., for weighing the cargo in a cargo yard), the monitoring of the occupancy of a section of a building (e.g., for determining the load experienced by a section of a building in order to obtain the number of people in the section so as to control the heating, cooling, ventilation and lighting in accordance with the section occupancy for the purpose of saving energy, or to monitor building evacuation), and the monitoring of the vehicles on a road (e.g., for weighing the vehicles, particularly trucks, on a road for the purpose of road damage prevention, traffic monitoring, and truck axle counting).
Weighing systems in the prior art have involved conventional weighing scales (US 20150175365, US 20110238210) and load cells (IN 2014MU02525, US 20090292427, US 20160243610). A common type of weighing scale uses a spring and functions by balancing the weight due to gravity against the force on the spring. Another common type of weighing scale uses a balance beam, which functions by balancing the weight due to the mass of an object being weighed against the weight of a known mass. Load cells (e.g., hydraulic load cells, pneumatic load cells and strain gauges) are transducers that give an electrical signal that is proportional to the weight being measured. The weighing scales and load cells are limited in the weight range (particularly in the high end of the range), ruggedness, and applicability to the weighing of objects of large volume (such as the load experienced by a section of a building).
Self-sensing refers to the ability of a structural material to sense its own condition without the need for embedded or attached sensors. In other words, a self-sensing structural material is multifunctional material that is capable of both structural and sensing functions. Such a material is also said to be intrinsically smart.
Relevant attributes to be sensed include stress (which relates to the force), strain (which relates to the dimensional change), damage, temperature, etc. Stress and strain are linearly related to one another in the regime of elastic deformation. The self-sensing in cement-based materials is useful for the monitoring of bridges, highways, nuclear reactors, underground spaces, oil and gas wells that involve cementing, and deep sub-surface storage of natural gas or carbon dioxide. The sensing is preferably fast enough (short enough in the response time) that it is suitable for real-time monitoring.
Stress sensing relates to force sensing, which is pertinent to load monitoring and weighing. HVAC is the abbreviation for heating, ventilation, and air conditioning. It is directed at providing both thermal comfort and adequate indoor air quality. Weighing is needed for the monitoring of the HVAC-zone occupancy in buildings, so as to control the heating, cooling, ventilation and lighting of each HVAC zone of a building in accordance with the zone occupancy for the purpose of saving energy. Weighing is also useful for building evacuation monitoring. Weighing in highways and bridges is also needed for traffic control, the determination of the number of axles of a truck, the protection of highways and bridges from damage by overweight trucks, and city evacuation monitoring.
Strain monitoring is pertinent to operation control. Since dynamic strain occurs due to vibrations, strain monitoring is pertinent to structural vibration control. In normal structural operation, the strain is completely reversible and is in the elastic deformation regime. When the strain exceeds this regime, the strain is at least partly irreversible. The irreversibility means a permanent change in dimension. A permanent dimensional change is usually undesirable to the operation of a structure.
Damage monitoring relates to structural health monitoring and hazard mitigation. Damage tends to degrade the mechanical properties of a material. The mechanical properties are critical to the performance of a structural material. Therefore, damage is undesirable and can be hazardous. As a result, the ability to sense the damage is needed in order to assess the damage and consider repair. Due to the aging of the civil infrastructure, structural health monitoring is critically needed.
A change in temperature can affect structural operation. Excessively high temperatures can be due to a malfunction and can be hazardous. Thus, temperature monitoring is desirable for operation control and hazard mitigation.
The sensing of stress or strain is typically more subtle and more challenging than that of damage. In particular, the sensing of stress or strain in the elastic deformation regime is more challenging than the sensing of stress or strain in the regime that occurs when the elastic limit is exceeded. The deformation regime beyond the elastic limit is commonly known as plastic deformation. The high challenge of sensing stress or strain in the elastic deformation regime is because the strain and stress in this regime are typically small compared to those in the regimes beyond the elastic deformation regime. The regimes beyond the elastic deformation regime include the plastic deformation regime and the fracture regime. In general, the sensing of a small strain or stress is more challenging than that of a large strain or stress.
Sensing by numerous methods in the prior art is not effective for sensing strains in a cement-based material (particularly strains in the elastic deformation regime), though they may be effective for sensing damage, corrosion extent and/or ion concentration (e.g., chloride ion concentration) in a cement-based material. Examples of these methods include electrical resistivity measurement (WO 2015150463), piezoelectric sensing (US 20160266086), acoustic sensing (US 20150338380, US 20130036821), impedance measurement (US 20160054247), corrosion potential measurement (US 20130106447), radio wave sensing (U.S. Pat. No. 9,394,784), electromagnetic wave sensing (WO 2016019247), gamma radiation sensing (WO 2017030579), and fiber optic sensors (WO 2015118333). Sensing techniques that are amenable to the sensing of both elastic strain and damage are needed in order to provide information on the effects of the full range of mechanical deformation.
The strain in the elastic deformation regime is reversible, whereas the strain at fracture tends to be at least partly irreversible. As a result of the reversibility of the elastic strain upon unloading, the sensing of elastic strain requires a sensing output that is also reversible upon unloading. Otherwise, the sensing mechanism would work once only, i.e., being able to sense during the first encounter of strain or stress only. This requirement of reversibility adds to the challenge of sensing elastic strain or stress.
The attainment of sensing ability in a cement-based structure is most commonly achieved by embedding sensors in the structure. In other words, the cement-based material itself is not the sensor, i.e., it is not self-sensing. Examples of embedded sensors are fiber-optic sensors (WO 2015118333), piezoelectric sensors (US 20150338380), pressure sensors (US 20160201451), surface acoustic wave sensors (US 20130036821), temperature sensors (US 20150276702), microelectromechanical system sensors (US 20160266086), nuclear magnetic resonance sensors (WO 2015178883), radio frequency identification (RFID) tags (US 20140182848), integrated chips (WO 2016019247), and other sensors (US 20170160111, US 20170016874).
Compared to the use of attached or embedded sensors, the advantages of self-sensing include low cost, high durability, large sensing volume and absence of mechanical property loss. This is because structural materials are necessarily low in cost and high in durability. Attached sensors tend to be not durable, as they can be detached. Embedded sensors tend to degrade the mechanical properties of the structural material, in addition to be not amenable to repair or maintenance.
An admixture in a cement-based material refers to an ingredient other than cement, aggregates (e.g., fine aggregate in the form of sand and coarse aggregate in the form of gravel) and water that is included in the cement mix used to form the cement-based material. Examples of admixtures include polymer particles (e.g., latex particles), ceramic particles (e.g., silica particles), carbon particles (e.g., carbon black), and short fibers (e.g., short carbon fibers). Admixtures tend to be expensive compared to cement, aggregates or water.
Self-sensing has been reported in cement-based materials that have been rendered electrically conductive by the use of electrically conductive admixtures, such as carbon fibers (U.S. Pat. No. 5,817,944, WO 2017011460). The presence of a solid admixture in the cement mix tends to increase the viscosity of the slurry, thereby decreasing the workability of the slurry. The workability of the slurry is important for the fabrication of cement-based structures with various shapes and dimensions. In addition, the presence of a solid admixture tends to increase the air void content in the resulting cured cement-based material, due to the small air voids at the interface between the admixture and cement. An increase in air void content degrades the mechanical properties of the cement-based material. Furthermore, the admixtures are expensive, thus greatly increasing the cost of the cement-based material.
Self-sensing has also been reported in cement-based materials containing aggregate that has been coated with a polymer-based film containing carbon nanotubes (US 20160340245). The presence of the carbon nanotubes decreases the electrical resistivity of the cement-based material, but also decreases the elastic modulus (US 20160340245). The decreased modulus is partly because the bonding of the aggregate to the cement is degraded by the presence of the polymer-based film on the surface of the aggregate. In general, the bonding between polymer and cement is not adequate. This bonding is very important for the mechanical properties of the resulting cement-based material. Without adequate mechanical properties, a cement-based material is not useful as a structural material. Furthermore, the presence of carbon nanotubes increases the cost of the cement-based material significantly.
The electrical conductivity of a cement-based material enables the electrical resistivity (which is the reciprocal of the electrical conductivity) of the material to change with the condition of the material, thereby enabling the electrical resistance to indicate the condition. The phenomenon of the electrical resistivity of a material changing with strain is known as piezoresistivity, which must be distinguished from piezoelectricity. The piezoelectric effect involves energy conversion, but the piezoresistive effect does not.
Without the electrically conductive additive (whether the additive is an admixture or a coating on the aggregate), the cement-based material tends to be too high in the electrical resistivity for the resistance of a large volume of a cement-based structure to the effectively measured. In general, the measurement of a very high resistance is difficult, due to the need for a high voltage in order for an adequate electric current to flow in the material. Sources of high voltage are expensive and high voltage tends to be hazardous.
In the absence of a conductive additive, the electrical conduction of a cement-based material is dominated by ionic conduction rather than electronic conduction. The ionic conduction (and thereby the conductivity) is enhanced by the presence of moisture, chloride ions or other ionic species. The variability of the electrical resistivity with such chemical species makes the resistance change due to the condition (such as stress, strain, damage and temperature) not sufficiently clear or reproducible. However, in the presence of a conductive additive, the conduction is substantially electronic, such that the contribution of ionic conduction to the overall conduction is much reduced. As a result, in the presence of a conductive additive, the ability to sense a condition (such as stress, strain, damage and temperature) is less affected by the chemical species in the environment. On the other hand, in the absence of a conductive additive, the resistance can be used to indicate the concentration of chemical species in the form of ions in the cement-based material, due to the promotion of the ionic conduction by these ions (WO 2015150463).
Even in the absence of conductivity-enhancing chemical species in the environment, the electrical resistivity of a cement-based material without a conductive additive does not vary with the condition (such as stress, strain, damage and temperature) with adequate intensity or repeatability. This is due to the predominantly ionic nature of the conduction and the inadequate sensitivity of this conductivity to the condition.
Due to the abovementioned multiple reasons, the presence of a conductive additive is important for rendering a cement-based material to be able to sense its own condition effectively through the measurement of its electrical resistance. However, the conductive additive (whether particles, fibers or nanotubes) is very expensive compared to cement and the aggregate that is commonly used in concrete. Moreover, existing concrete structures rarely have any conductive additive in the concrete. As a consequence, the method of achieving self-sensing using conductive additives is not applicable to the vast majority (essentially all) of existing concrete structures.
The implementation of the resistance-based self-sensing involves the application of electrical contacts. The electrical resistance associated with an electrical contact must be small enough, so that it does not overshadow the resistance associated with the volume of the cement-based material. Thus, the electrical contacts must be high in quality, with the electrically conductive material (typically a metal) that makes up the electrical contact being in intimate contact with the cement-based material. Even if the resistance of the electrical contact is small, it may still vary as the condition (e.g., stress, strain, damage, temperature, etc.) changes. This means that both the resistance of the electrical contact and the resistance of the volume of the cement-based material change with the condition. The volume resistance is the quantity that is indicative of the condition being sensed. The variation of the contact resistance with the condition may cause the measured resistance (which includes both the contact resistance and the volume resistance) to be not indicative of the condition, thereby causing the sensing to be misleading. To alleviate this problem, four electrical contacts are used, with the outer two contacts for passing current and the inner two contacts for measuring the voltage. The resistance measured is this voltage divided by this current, and is the resistance between the two inner contacts. Because essentially no current flows through a voltage contact, there is essentially no potential (voltage) drop at each of the two voltage contacts. Therefore, the resistance obtained using four electrical contacts largely eliminates the contact resistance from the measured resistance. In contrast, the use of only two electrical contacts, with each contact serving for both current passing and voltage measurement, causes the measured resistance to include the contact resistance. In spite of the superior reliability of the method involving four electrical contacts compared to the method involving two electrical contacts, the former makes the implementation of the technique more difficult. In other words, installing four electrical contacts to measure the resistance of a segment of a cement-based structure is much more inconvenient (more labor intensive) than installing two electrical contacts.
The method of embedding two electrodes in a cement-based material for electrical measurement has been taught (EP 2947456). The two electrodes are metals that can be the same in composition or different in composition. An electrode can be in the form of a relatively inert metal such as titanium. It can also be in the form of a steel component (such as a steel reinforcement) that is commonly present in a cement-based structure anyway. The drawback of using two electrical contacts has been discussed above.
The alternating current (AC) impedance differs from the direct current (DC) resistance in that it is a complex quantity that consists of a real part (the resistance) and an imaginary part (the capacitance and inductance, with the capacitance being more relevant to the subject of this disclosure than the inductance).
The impedance depends on the AC frequency. The variation of the impedance with the frequency can be analyzed in terms of equivalent circuit models for describing the electrical behavior. The analysis typically involves the fitting of the curve in the Nyquist plot (plot of the imaginary part of the impedance to the real part of the impedance for various frequencies). However, the equivalent circuit model obtained by the curve fitting is not unique. As a consequence of the non-uniqueness, the values of the circuit parameters (resistances and capacitances) in the circuit model are only meaningful in the context of the particular circuit model and are not generally meaningful.
Because the impedance includes the resistance as its real part, the measurement of the impedance involves the same issues as mentioned above in relation to the measurement of the resistance. An issue pertains to the abovementioned requirement that the electrical contacts are associated with low values of the contact resistance. Another issue pertains to the abovementioned need for using four electrical contacts rather than two electrical contacts.
The measurement of the capacitance has its issues too. An issue pertains to the fact that the impedance meter (often called an LCR meter) is not designed for measuring the capacitance of an electrical conductor. When an impedance meter is used for testing a conductive material, the capacitance value that it outputs can be off from the true value by a large amount (even off by orders of magnitude). Depending on the composition and ion concentration, a cement-based material can be conductive enough for this issue to be pertinent.
The parallel-plate capacitor geometry is commonly and classically used for measuring the capacitance of a material that is sandwiched by the two facing plates (i.e., two conductor plates commonly referred to as electrodes). The capacitance is in the direction perpendicular to the plates. Due to the small thickness of the material being tested between the two plates and the large area, the capacitance can be rather large. Thus, this variation of the parallel-plate capacitor geometry is effective for obtaining information that pertains to the capacitance. On the other hand, due to the small thickness and large area, the resistance can be rather small, though the value depends on the resistivity of the material.
In a less common variation of the parallel-plate capacitor geometry, the material being tested is positioned between the parallel proximate edge surfaces of two coplanar plates (EP 3115781). In other words, the material is sandwiched by these edge surfaces, so that the thickness of the sandwich is large and the area of the sandwich is small. The capacitance measured is in the direction perpendicular to the two edge surfaces. This geometry tends to be associated with a small capacitance, due to the large thickness of the material being tested between the two edges and the small area of the capacitor. Thus, this variation of the parallel-plate capacitor geometry is not effective for obtaining information that pertains to the capacitance. On the other hand, due to the large thickness and small area, the resistance tends to be rather large, though this value depends on the resistivity of the material.
A parallel-plate capacitor actually involves three capacitors in series electrically, whether the electrodes are facing or coplanar. The three capacitors that are electrically in series consist of the capacitance of the sandwiched volume of the material being tested, and the capacitance of each of the two interfaces, with each interface being that between the sandwiched material and one of the two electrodes. The well-known equation for capacitors in series is1/C=1/C1+1/C2+1/C3,  (1)where C is the overall capacitance of the three capacitors (with capacitances C1, C2 and C3) in series. Hence, the measured capacitance C of the parallel-plate capacitor is given by1/C=1/Cv+2/Ci,  (2)where Cv is the capacitance of the volume of sandwiched material and Ci is the capacitance of one of the two interfaces. Thus, neglecting thereby assuming that C=Cv, can result in an incorrect determination of Cv from the measured C.
The relative electric permittivity is a material property that reflects the degree of damage or the ion concentration in the material. For example, the permittivity of a cement-based material increases with the chloride ion concentration in the material, thereby allowing the permittivity to indicate the chloride ion concentration (EP 3115781).
The relative permittivity κ is obtained from Cv using the well-known equationCv=εoκA/l,  (3)where εo is the permittivity of free space (8.85×10−12 F/m), A is the area of the sandwich (i.e., the area of each electrode, which is the same as the area of the sandwiched material being tested), and l is the thickness of the material sandwiched by the two electrodes. Without a reliable determination of Cv, κ cannot be reliably obtained by using Eq. (3). Specifically, neglecting the term 2/Ci in Eq. (2) causes 1/Cv to be overestimated, thus causing Cv to be underestimated, and causing κ to be also underestimated.
Capacitive sensing is important for touch sensing, as needed for touch screens, which are commonly used in electronic devices such as computers. Touch sensing is based on the concept that the human finger is an electrical conductor and its contact with an electrical circuit changes the capacitance of the circuit. In connection with touch screens, a large variety of electrode patterns and associated circuits have been taught (US 20170269779, US 20170024033). However, such sensing systems are not capable of and not applicable to the sensing of the force exerted on a cement-based structure. The use of such concepts for the sensing of the force exerted on a cement-based structure would be very expensive, due to the electrical circuit. In addition, the durability of the circuit under substantial mechanical force is low.
The embedment of electrodes in a wet cement-based material (i.e., cement-based material that is either only partly hydrated or not yet hydrated) for measuring the electrical impedance of the wet material as a function of the frequency has been taught for the purpose of obtaining information on the physical properties of the wet material (US 20160054247). Due to the high ionic conductivity resulting from the water in the wet material, the conductivity or impedance of the wet material is very different from that of the dry (hydrated) material. Although the knowledge of the physical properties of the wet material is useful for understanding the basic science of the wet material, the teaching is not directed at sensing the condition of the dry (hydrated) material. The hydrated (dry) state is the state in which cement-based structures are used, so it is practically more important than the wet state, which is important only for the process of installation of a cement-based slurry in a structure.
The measurement of the interaction of gamma radiation (which is high-energy electromagnetic radiation) of various frequencies with a cement-based structure can provide information that reflects the condition of the structure (WO 2017030579, WO 2016148696). However, this method requires a gamma radiation source and instrumentation for detecting and analyzing the interaction of the gamma ray with the cement-based structure. Both the source and the instrumentation are expensive. In addition, the gamma radiation (even more energetic than X-ray) is hazardous.
Piezoelectric cement-based materials are commonly obtained by incorporating a piezoelectric material (commonly in the form of ceramic particles such as lead zirconotitanate, which is abbreviated PZT) in the cement-based material. The incorporation is aimed only at damage sensing (WO 2008094358), salt corrosion resistance enhancement (US 20080179993), and vibration reduction (US 20110252734, US 20110252715).
The direct piezoelectric effect converts mechanical energy to electrical energy. Thus, in response to mechanical energy input, electrical energy output occurs. The electrical energy output manifests itself as the generation of voltage and/or current. Hence, this effect can be used for the sensing of the mechanical energy input through measurement of the electrical energy output. For a static force input, the output is a static open-circuit voltage or a pulse of a close-circuit current. The static open-circuit voltage can be measured using a voltmeter. However, due to its short duration, the current pulse requires for its measurement an instrument that is capable of rapid real-time response. An example of such an instrument is an oscilloscope, which is a type of electronic test instrument that provides plots of constantly varying signal voltages as a function of time. As a result, the measurement of a current pulse is relatively demanding in the instrumentation. For a dynamic force input, the electrical output is dynamic for both voltage and current, as can be measured by using an oscilloscope. Therefore, the measurement of the dynamic electrical output is relatively demanding in the instrumentation.
The measurement of the impedance or capacitance requires an impedance meter (LCR meter). The higher the frequency, the more expensive is the meter, and the less effective is the sensing of chemical species that cannot respond to rapid changes in the polarity of the AC electric field used for the impedance or capacitance measurement. Examples of such chemical species are ionic species (e.g., chloride ions) and molecular species. Cement-based materials contain ions, notably calcium ions. Chloride ions are undesirable in cement due to their promotion of the corrosion of the steel reinforcement that is commonly present in a cement-based material. Thus, the effectiveness of a technique that operates at a low frequency is desirable. Relatively high frequencies of 16 kHz (1 kHz=1000 Hz) or above are used in prior work (EP 3115781).
The present invention is directed at overcoming these and other deficiencies in the art.